Aqueous extracts after ultrafiltration of lymphoid tissues, particularly spleen, contain a macromolecular inhibitor which is totally cell-specific for lymphocyte transformation. This specific endogenous, and noncytotoxic protein or chalone will inhibit the rejection of skin transplants in vivo. About half of the lymphocyte chalone activity in our splenic ultrafiltrates is derived from a unique bacterium (small G-negative rods) which only at 4 degrees Centigrade will accumulate on its surface an apparently cellspecific and noncytotoxic inhibitor of lymphocyte transformation produced either by lectin or in mixed lymphocyte culture. We are attempting to prepare quantities of this material for isolation and purification studies. BIBLIOGRAPHIC REFERENCES: Attallah, A. M., Sunshine, G. H., Hunt, C. V., and Houck, J. C. The specific and endogenous mitotic inhibitor of lymphocytes (chalone) Exp. Cell Res. 93: 283-292, 1975.